It took off from St Trond aerodrome near Brussels, in occupied Belgium, on the morning of Monday Aug 26, 1940.

With 16 33lb bombs on board, it was one of nine German aircraft from the squadron on a mission to attack a Fighter Command airfield at Manston, north Kent. But the raid was a feint – designed to lure British aircraft into the air where waiting German fighters could pounce on them.

The four young men on board were the pilot, Feldwebel (equivalent to an RAF Flight Sergeant) Willi Effmert, a married 24-year-old, from Bad Salzuflen, a spa town near Hanover; observer, Unteroffizier (Sgt) Hermann Ritzel, 21, from Frankfurt am Main; wireless operator, Unteroffizier Helmut Reinhardt, 27, from Bochum; and flight engineer Gefreiter (Cpl) Heinz Huhn, 21, from Lotterfeld, then in East Prussia, now Loznik, in Poland.

As the formation made its way across the sea, its presence was detected by the RAF warning systems, and 264 Squadron, based in Hornchurch, Essex, among other units, was sent to intercept over Herne Bay.

In the cockpit of one of the squadron’s Defiants was Pilot Officer Desmond Hughes, with his gunner, Sgt Fred Gash.

At the time of his death, aged 72, in 1992, Hughes, who later rose to Air Vice Marshal was writing his memoirs – still unpublished – which describe in dramatic detail the moment he and his comrades intercepted the German bombers, at 15,000ft above the north Kent coast.

He wrote: “The specks grew into the long pencil-slim silhouettes of Dornier 17s and suddenly, there were the black crosses, insolently challenging us in our own back yard!”

That day, Hughes and Gash were credited with downing two Dorniers, and the RAF Museum researchers believe one of them was the aircraft recovered last week.

He described the first “kill” in greater detail, as his squadron, mindful of the nearby escorts, came in beneath the Dorniers, to attack them at their most vulnerable point.

“Fred Gash took as his target the second Dornier and made no mistake – his De Wilde incendiaries twinkled all over it but particularly on its engine.

"It began to fall out of the formation, the hatch was jettisoned, two parachutes streamed as little dark figures bailed out and the stricken aircraft went down increasingly steeply with its starboard engine well alight.”

The second “kill” occurred in the chaotic scenes that followed the initial attack. Hughes wrote: “Fred had been blazing away at another Dornier (which he later reported as having 'brewed up’).”

The Defiants were then attacked by German Messerschmitt Bf109s. After shaking them off, Hughes and Gash headed back to base, where they discovered six bullet holes in their aircraft. That night, Hughes sent a telegram to his parents. It said simply: “Two up and lots to play.”

Back on board the Dornier, the gunfire had hit both engines and the cockpit, causing injuries among the crew. Ritzel lost two fingers on his left hand.

With at least one engine stopped, Effmert attempted to make a controlled landing on Goodwin Sands, six miles off the Deal coast, which can be exposed at low tide. As it came in, the stricken aircraft appears to have somersaulted and settled into the water on its back.

The remains of the World War II Dornier bomber (PA)

Two of the four, Effmert and Ritzel, were recovered. The other two died, with their bodies washing up on different sides of the North Sea. Reinhardt was buried in Ysselsteyn, Holland, while Huhn was interred in the German war cemetery in Cannock Chase, Staffs.

The survivors became prisoners of war and were transported across the Atlantic to camps in Canada. At the end of the war, both returned to Germany.

Effmert found that his wife had been killed during an Allied bombing raid. He is understood to have remarried several years later and died about 15 years ago.

Ritzel married, in 1949, to Annemarie, and had a daughter, Gertrud. The couple divorced in 1964 and he remarried, in 1976, a woman named Trude. He worked as an engineer and lived in Fulda, in central Germany. He died in 1996.

His grandson, Christian Nowak, 38, said his grandfather had not been a member of the Nazi Party before the war and had joined the Luftwaffe because he had experience flying gliders.

“He had a normal life. He liked to go hiking in the Alps. He liked to draw pictures, and he spent a lot of time in his garden – he liked spending time outdoors. He never flew again. I think that he was unable to fly a glider because of the injury to his hand.

“After the war, he was an absolute opponent of war. But at the time, in an air battle – it’s you or him – and you have to save your own life. The goal is to survive.”

He added: “I don’t think of him with pride because of his wartime experience, but I am proud of him for other reasons – he was an engineer, and he discovered a few things in the area of electronics, which he patented. I am proud of him for that.”

Hughes ended the war as the third highest scoring RAF night fighter pilot, nicknamed “Hawkeye” Hughes by the press and known for taking his pet dog on a sortie. The animal, Scruffy, survived the mission but was run over and killed shortly afterwards. Hughes finished with 18 and half “kills” – the “half” was a bomber shared with another RAF pilot. He had been lucky, when downing 5K + AR, to have found it flying apart from its fighter cover.

The Defiant with its rear gun turret was slow and vulnerable to anything other than an undefended bomber. That week alone, his squadron lost nine air gunners, five pilots, one commanding officer, an acting CO injured and two flight commanders shot down. At one point, the most experienced pilot left was a pilot officer not yet 21.

Paddy Hughes, 70, his son from Templecombe, in Somerset, said: “The Defiants got a few enemy aircraft but also got a terrible battering.”

The Dornier – the identity of which has yet to be finally confirmed – was transported from Ramsgate to RAF Museum Cosford, Shropshire, on Saturday.

Over the next 18 to 24 months, the museum’s experts will use pioneering conservation techniques to try to turn the corroded, battered aircraft into what it would have looked like during the Battle of Britain.